Ethylene purification



J. M. FOLZ Filed Oct. 15, 1957 June 2l, 1960 ETHYLENE: PURIFICATION tance in the potro-chemical industries.

2,942,042 Patented June 21, 19d() 2,942,042 ETHYLENE PURIFICATIGN John M. Folz, Bartlesville, Okla., assigner to Phillips Petroleum Company, a corporation of Delaware Filed oct. 1s, 19s1,ser.N0. 690,296 s claims. ((11.260-677) This invention relates to the purification of ethylene. In one aspect it relates -to the treatment of ethylene containing impurities such as water, acetylene, carbon monoxide, carbon dioxide and oxygen to remove said impurities and provide a purified ethylene product. In another aspect the invention `relates to a method of operating a process for treating ethylene to remove contaminants therefrom.

Ethylene in recent years has found increasing impor- For example, it is frequently used as -a chemical intermediate in the preparation of such materials as acetaldehyde, acetic acid, ethanol, acetylene, ethylene glycol, ethylene oxide, etc.

By far the largest use of ethylene, however, both present and prospective, lies in the polymerization field and specitically in the catalytic polymerization of 4ethylene to normally solid polymers. Ethylene, while naturally occurring in petroleum, is obtained principally by the dehydrogena-tion of low-boiling paraffin hydrocarbons such as ethane, propane, butane, etc. In -the process of-manufacture of ethylene from parains various side products are formed in small quantities. These include such materials `as acetylene, carbon monoxide, carbon dioxide, and

oxygen. Due to the sensitivity of the various polymerization catalysts to these materials it is desirable when utilizing ethylene as a feed material in the polymerization process to first remove -any side products.

lrrone process the `removal of undesirable compounds from the raw ethylene is provided by passing the feed material through a series of treating operations in which the ethylene is `contacted with various chemical treating agents. Some of the chemicals used are expensive and require extensive treatment, involving heating and cooling operations, to eect their recovery for reuse in the process. The over-all purification process thus requires a substantial expenditure of thermal energy dueto the large .for increasing the efficiency of purification of ethylene to remove contaminants therefrom.

Still another object of the invention is to provide a` process for the purification of ethylene in which heat exchange and treating operations are carried out at maxi-` mum efficiency. Y Yet another object of the invention is to provide a process for the purification of ethylene wherein uctuations in the quantity of desired product do not effect the eliiciency of the treating process. l These and other objects4 of the invent-ion will become more readily apparent from the following detailed de` scription and discussion.

, The foregoing objects are attained broadly by operating theethylene purification process such'that the treating materials which are utilized in heat. exchange are maintained at a substantially constant flowA rate and` recycling excess puried ethylene to the feed' material whereby the ethylene feed rate is maintained substantially constant.

As used herein the term substantially constant means constant, within the capabilities of conventional ow and control instrumentation.

Theinvention is best described by reference to the accompanying drawing which is a diagrammatic illustration of an ethylene treating and purification unit comprising a carbon monoxide removal column, and acetylene absorber, water wash columns, a carbon dioxide -removal` column, and oxygen removal column, inlet and exit dryers and associated heat exchange equipment, conduits, etc, suitable for carrying out the invention.

Referring to the drawing, raw ethylene feed prepared by thepyr'olysis of ethane is passed through a dryer for the removal of water. In addition to water lthe ethylene feed contains, as impurities, quantities of acetylene, carbon monoxide, carbon dioxide and oxygen. Removal of water from the ethylene feed is provided by contacting thismaterial with a suitable drying agent, such yas bauxite, alumina, silica gel etc. Since the drying operation is customarily carried out as a batch process, it is desirable to provide a plurality of dryers whereby one or more of the 'dryers can be regenerated, emptied, filled, etc., while another dryer is in operation, thereby providing for continuous drying of the ethylene feed. In this particular operation dryers 4 and 4a are provided with the feed material being passed through dryer l4, with dryer number of process streams which must be heated or cooled p process can fluctuate substantiallyovera period of time.,

Customarily the treating process is operated to supply only the required -amount of purified ethylene; however, in viewof the interrelationship of the various heat exchange operations it is not possible to obtain ,the desired y purification if any of the process streams are varied to any substantial degree.

It is an object of this invention to provide an improved process for the purification of ethylene.

Another object o f theinvention is to provide a process 4a in standby condition. While drying can be carried out over a rangeof temperature `best results are provided at lower temperatures and preferably this operation is carriedout at normal atmospheric temperatures.

Afterleaving dryer 4 the ethylene feed, which is substantially freed from water, is passed through conduit 8 and compressor 10 and introduced to carbon monoxide removal column 18. This column can be a conventional fractionating column containing `trays or baflies, or can bea packed tower. In this particular example the tower is packed with Raschig` rings. In order to effect removal of carbon monoxide from the ethylene yfeed it is necessary that the temperature in column 18 be reduced to below the boiling point of ethylene so that this material is condensed. T he desired reduction in temperature is effected infpart by passing the ethylene feed through exchangers 12 land 16 and in part by reuxing the column with overhead material which is reduced in temperature by propane refrigeration in condenser 22, It is possible to operate over a `range of temperature in the CO removal column by varying the pressure therein, with the only limitation being that the temperature is sutiicie'ntly low to maintain the ethylene in the liquid state.` Usually this column` is operated with a top temperature" between a en'ter an accumulator 24. The liquid condensate from the accumulator is recycled 4to the column through conduit 28 as refluxand uncondensed carbon monoxidegas is withdrawn from conduit v26. iThe tower ,bottoms which comprise the ethylene 'feed,no'w substantially freed from -water andcarbon monoxide, arerem'oved through conduit 30. A ,portion of the bottoms are returned to the column through Iconduit A'372, andexchanger 16 and serve 'to reboil 'the column with'heat picked upv from the ethylene feed. The remainder of the column bottoms -are passed through exchanger 12 in heat exchange with the ethyleneV feed then through exchanger '3'8 wherein they are further heated in indirect heat exchange with '-the overhead from l.the dimethylformamide absorber 40 and are thereafter introduced to said absorber.

Before entering exchanger 38 the bottoms are joined by puried recycle ethylene through conduit `144. The two streams AVarev proportioned so that the quantity of material passing through exchanger 38 and into the absorber remains substantiallyconstant at'all times. The amount of recycle ethylene passing through conduit 144 depends on the quantity of purified ethylene 'withdrawn as product through valve 143, thus as the ethyleneproduct increases or decreases a corresponding change in flow of recycle ethylene results. The total ethylene feed to exchanger 38 passes lthrough an orifice 5 'towhich there is connected a flow control 36 which inA turn it connected to a control valve '34 in conduit 3i). When 'the ow rate through conduit 144 varies the flow through orifice I5 also varies. The latter change in flow `is transmitted to ow controller 3'6 'which in turn transmits a signal to control valve34 which opens or closes .to adjust 4the iow rate Vthrough conduit 30, whereby the 'flow rate through orifice 5 is returned to its .previous level. Thus, the total ethylene feed rate is maintained .substantially lconstant immaterial of variations in the :net yield of purified ethylene product. Y vAbsorber 40 is .utilizedto effect removal of acetylene from the ethylene feed. This vessel can be a conventional tray-type fractionat'ing tower 'or can contain various 1types of packing materials. In this .specific instance, the .absorber isa tray .type vessel. The ethylene'feed Yentering the absorber is contacted with -dimethyl formamide (DMF) which-servesras an absorbent :for acetylene present .in said feed.v The feed is introduced in the bottom portion of the7^ absorber andpasses upwardly countercurrent -to the 4DMF which enters the absorber Athrough conduit 96. Since absorptionis aided yby low temperatures this process Ais preferably carried but inatemperature range below atmospheric, usually .between labout -20 and about 56 F. Elevated pressures 'are necessary in order to retain fthe acetylene Yin 'the absorbent, for example between about 275 :and about 325 p:s.-i.g. 'The quantity l'of labsorbent required 'to effect removal 'of the acetyleneis dependent on lthe amountof acetylene'present in the ethyleneffeed'and Ausuallyis maintained at 'a rate between about "75 and about 1'75 "pounds per pound of acetylene. l ,v l

ln' passing throughthe absorber the dimethylforrnamide absorbsnot only-acetylene but also a portion -of the ethylenefrom thefeed. Recovery "of lthe latter material is ,provided by passing theabsorber bottoms through; a primary `dash drum S4 and a secondary flash drum 58 wherein successive reductionsinpressure are made. The gases 'which are released from the v'flash drums-are concentrated fin ethylene Vand are Vreturned to the absorber through conduits 62 and-64, and compressors '60.and66. The p'r'imary dash drum 'is usually 'operated at apressure in "the range Vof rbetween about '50 and about 75 p;'sfi.a.

and the secondary flash drum is operated between about 0 and about 30 p.s.i.a.

The liquid from the secondary flash drum, which comprises DMF containing acetylene, is removed through conduit 68, passed through exchangers 70 and 72 wherein it is heated in indirect heat exchange with two DMF streams and then introduced vto stripper 74. The heat added to this stream acts Yto release acetylene from the DMF and gas passes overhead'rthr'ough conduit 76 and cooler- 78, is .then contacted in indirect .heat exchange with overhead from theiDMFabsorber and further cooled in exchanger 43 whereby any DMF carried overhead from the stripper is'condensed. The fmixtre Iof condensate and gas enters fa knock-out drum 180, from which the acetylene is Withdrawn through conduit 82. The liquid from the knock-out drum is returned to the DMF stripper through conduit 84 and'acts asa reflux material. The DMF` `stripper isrusually operated at a pressure inthe same range as the secondary dash drum-and at a temperature of between about 125 `and about 400 .F. stripper bottoms, comprising substantiallyl 4pure DMF are withdrawn through conduit 86 cooledin indirect heat exchange vwith secondary flash drum lbottornsin exchanger 72, Afurther cooledin water cooler 90 andintroduced to DMF surge tank 92. This material, which Vis combined with any required makeup DMF introduced through conduit f94, comprises the absorbent feed to DMF kabsorber 40. As required @the DMF is withdrawn from .the surge tank -through Aconduit ,96, cooled in Aheat exchange with the secondary flash drum bottoms in exchangenlth passed through exchanger '50, in indirect yheat exchange with lthe 'DMF absorber overhead, .for further cooling and finally cooled in propane exchanger 98,v after which it .is introduced to the DMF absorber through conduit Y 96as`,previously noted. Y K t v The .DMF absorberoverhead vaponjwhich comprises ethylene feed substantially free ofrwater, carbon monoxide and acetylene passes into .a knock out :drum44 Lwhere any entrained liquid is removed, the `latter being returned to the absorber through conduitAS. The gases Afrornthe knockout drumare passed throughconduit 46, exchanger Sgexchanger 38,. and exchanger 59, after which theyarc compressed fin compressor 99 and introduced to a-series of towers for the removal of` othercomponents -Tfrom the ethylene feed. These towers comprise `a primarywash towerfl'tl), .followed by Vacarbon 4dioxide removal tower 108, an oxygen Aremoved tower H6, a linal `water. wash tower 124, =a `knock out drum 132 and driers v140 and 140A. 'Duringfpassage-of the ethylene gas `through these towers the primary Water wash is .utilized .to remove DMF, caustic is used to remove carbon dioxide, pyrogallol :is -used .to remove oxygen, thefflnal water washgis used to remove pyrogallol and the driers serve to remove any water present in thepuriiied ethylene product. The fma- Y terial 4leaving the drierstis divided into -twoporitions vwith one portion being yielded throughl yalve 1437as product and the other lbeing recycledto the ethylene feed lthrough conduit :144. Y -It isnecessary that fthe `carbondioxide and oxygen removaloperations be carried yout at anelevated pressure in order to `retain `these contarninantspfiny the Y treatingmaterials -in which'gthey are absorbed. Usually thesetreating operations are carried out-at aipressure .in the :range of 'between about 50.0 iand about..575 pzsi. Since Vincreasing 'temperatures :require :more elevated *pressures it `isdesirable rthat the carbon zdioxide and oxygen removal `be -carried out at 'low temperatures :usually ynot substantially greater Vthan that about atmospheric temperature'. The* quantities yof treating agents err'ip'loyed depend on the 'amount 'of contaminants whiehniust be removed from 'the ethylene. 'Inthe case 'of'carbon dioxide removal the caustic isA preferably introduced 'at 'a 'rate'of between 'about 45 and about 17"5 4'pounds'per pound of carbon dioxidepresent in' l't'h'e ethylene feed. In fthefoxy-v gen removal step-the pyrogilel is prefe'riy maintained mi rate between 'aeom'fzfs t and jabon-T5600. pounds, 'pefpoud f'xygen present inthe yethylene stream. L VIn"v each f of the.. preceding operations the f, quantity' .of

pMF'employed maintained sbstamany constant arid l i the total ethylene ifed' is4 m'aintafufnecl` substantially constantl` by suitably pfroportioningthefresh-feed-.and recycle ethylene. 4, The :precedingdiscussionqhasbeen-directed `to a -preferred embodiment of the invention; however,` this is not M lp'o'se'ofi the-'invention-it spossible toV `employ treating lageir'tsother 'thanfth'ose' specicallyset forth in the dislcussinof' thedrawin'g. "'orexample removal' ofV acetyl- "ene fromitlreethylenefee'd 'canfbe carried lout by using 'conventional absorbent materials, Such as acetone,

butyrlactoe, 'Zipy'rollidone "et`c., removal of carbonm monoxide 'by the use of copper ammonium acetate or formate, and the `removal of carbon dioxide by: the use of mono, dior triethanolamines,potassium-carbonate, .-portassium lhydroxide, sodium Ihydroxide, etc.

'Ilhefprocess of this invention provides nadvantages over..`

the.` previous Amethods of operation in thatV it is possible, -rsince-thefflow' rate of the ethylene feed'and the DMF ab- -sorbent larecmain'tained constant, to design 'vessels-heat exchangers, pumps, etc., 'tooperateatoptimum eflieiences.

-It s'falso'possibleby virtue of maintaining the various 110W rates constant to provide maximum removal of `contaninants `and side 'products fromthe ethylene feed.`

Theethylene treated inthe method of 4this invention `is hydrocarbons 'such'as' ethne,"propane, butane and mix- 'furesxhereoa The'iection is carried out at elevated terperturesjfor example, when ethane is utilized as"a `feed material the reactiontakes place in the Vrange -of 1400 'tol 16'0`O`"F. When` higher boiling hydrocarbons are pyrolyzed it is found that the reaction proceeds at somewhat lower temperatures. Usually itis desirable to operate with low.pressures since higher `pressures have an adverse effect `on the reaction equilibrium, The reaction proceeds rapidly' 'therefore' it is preferable to maintain a short `rea`ction""rsidence time, i usuallyV between about 0.'5'uya`nd` about 0.8 sec. uIn orderto minimize coking adaidin obtaining thedesired residence time,` it is 'customary' to dilute thenfeed material .with an inert gas,

such as steam.

The following example is presented in illustration of a commercial application of one embodiment of the invention.

Example Flows Lb./hr. Raw ethylene feed (2) 3500 Composition:

(22H4 wt.percent 95.89 C2H2 wt. percent 1.40 CO p.p.m 24 CO2 wt. percent-- 0.29 CH4 wt. percent 0.21 CZHG wt. percent-- 2.21 Recycle ethylene (144) 100 Feed to `CO removal (8) 3600 Composition:

(22H4 wt. percent 95.81 C2H2 wt. percent-- 1.40 CO -p.p.m 23 CO2 wt. percent-.. 0.30 CH4 wt. percent 0.22 (22H6 wt. percent-, 2.27

amante V.-eed to acetylene removal (30) nn-.aaw-i-w---GGQO Composition: C2H4 wt.percent= -95t83 C2H2 wtiA percent .xL-43 CO2 wt.,percent= 3031 CH4 wt`percent 0.15 p 02H6. ;wt.percentr ,-2,28 DME to absorber (96) aa -6215 10 Composition:

' C''H'4- c :wt:'percent `l$9.11

`CH6 ;wt."percent l0.22 yCi-14;.; vvt:percent..A 0202 ACO2 :wtpereent.. "0103: 1 IDM'F WLpercent 9062" `fFedto'lC02-'rernovl (1114) 3500 Composition: C2H4 'wt, percent-- '97:20 "C02 wt: percent-- 10.31. wt.'percent 0115 C2H6 '.wt.percent [2:34 Feed to O2' removal (114) .."3490 Composition: 'Caldi :wt.'percent "9740 C114 wtipercent-- '.01'1'5 f Cz'H'S wt;`perc`ent 2.34 A'Water ;wt.percent "0111 1 Feed-.tofdriers ('138`) 1 349 Composition: y `^CH4 wt.percent-- 9`740 lCH4 vvt:percent-- 0115 `C2GH Wtpercent "2134 y p A H2 O Wtlpercent k0.1lf` "Net*ethylene iproduct 4 `3'3'86 f Composition: p

1C2H.1 vvt'percent.;97.5'1` `-"C`I1I4 wt."per'cer1t f `0.1`j

02H6 wtrpercehtg. ,2:34 f .40 i n l j lV Y [Temperatures FF. "Raw'ethylenefee'd (8) y `70 Coremovlcolnmn'(18): t y Q --L-*m.-"F';:fFPr-FmrlBottom t a 215 1combinedfeedfto'exhangerlss 7o 1CombinedjfeedftdDMF' absorber 40 56 DMFfbsorb'erx-Lw): p t

Top l .J jo Bottom--- L( 138 Primaryfnashfdmmrsn "11 Secondary flash drum (58) 4 Secondary flash drum bottoms:

To exchanger 70 4 To exchanger 72 94 To DMF stripper 74 l 15o DMF stripper (74):

Top 150 Bottom 346 DMF stripper overhead to exchanger (98) 150 KO drum (80) 10 DMF stripper bottoms to exchanger 90 288 DMF surge tank (92) 100 Surge DMF: Y

To exchanger 50 29 To exchanger 98 24 To DMI;` absorber 40 20 DMF absorber overhead:

To exchanger 38 2 To exchanger 50 13 To primary water wash column (100) 100A Primary Water wash column (110) `100 CO2 removal column (108) 100 Oxygen removal column (116) 100 Final waterwash column (124) 10o Driers (140 and 140a) 100 CO removal column (18) 1 310 DMF absorber (40) 300 DMF stripper (74) 20 Primary flash drum (54) 65 Secondary ash drum (58) 20 Primary water wash column (100) 550 CO2 removal; column (108) v 550 Oxygen removal column (V116) 550 Final water Wash column (124) 534 ,tions are within the scopeV of the invention. l

I claim: 1. In a process wherein ethylene is purified in a purication zone and, in conjunction with the purification, is

passed through a heat exchange system wherein it is subjected to indirect heat exchange with at least one material which has passed through said purication zone, so that variation'of the rate of flow of ethylene has anradverse eiect upon the heat exchange, and the amount of nally puried ethylene withdrawn as product varies, the improvement whichV comprises recycling, to said heat exchange zone, that part of the nally puried ethylene not withdrawn as product, and proportioning thus recycled ethylene, with respect to ethylene feed entering the process, to maintain substantially constant thetotal low rate of ethylene passing throughsaid heat exchange system.

2. In a process whereinethyleneis purified in a puri- Iication zone by contact with an absorbent which selectively dissolves impurities from said ethylene, said absorbent being Asupplied to said purication zone at a substantially constant rate, and, in conjunction with the purificatio'n,the ethylene is passed through asheat exchange system'whe're'ii it is subjected to indirect heat4 exchange *with 'ethylene withdrawn from said purification zone after contact with'absorbent vand subsequently subjected to indirect heat exchange with said absorbent prior to entry of said absorbent into said purication zpne, so that variation of the'rate of flow of'ethylene has an adverse effect upon the heat exchange, and the amount of nallypuritied 'ethylene withdrawn as product varies,jthe"improvement which comprises recycling to said heat exchange system, that part of the linally purified ethylene not withdrawn as product, and proportioning thus recycled ethylene, with respect to ethylene feed entering the process, to maintain substantially constant the total flow rate of ethylene passing through said heat exchange system.

t 3. A process according to claim 2 wherein said absorbentis selected from a group consisting ofcopper ammonium Aacetate and copper ammonium formate and the impurity absorbed is carbon monoxide.

4. A process yaccording to claim 2 wherein said absorbent is a basic reagent selected from the group consisting of monoethanolamine, ,diethanolamine, triethanolamine,

potassium carbonate, potassium hydroxide, and sodium hydroxide, and the impurity absorbed is carbon dioxide.

5. In a process wherein ethylene is purified in a puri- 'cation Vzone by contacting with dimethylformamide which 20A selectively dissolves acetylene from said ethylene and, in conjunction with the purification, the ethylene is passed through a heat exchange system wherein the puriied ethylf ene stream withdrawn from contact with said dimethylform-amide is subjected to heat exchange with acetylene recovered from solution in said dimethylformamide by stripping, then subjected to-heat exchange with the impure ethylene feed entering the system, and thenV With said dimethylformamide entering the purification zone, said dimethylformamide being supplied toY said p'urication zone at a substantially constant rate, in which heat exchange system variation of the rate lof flow of ethylene will have an adverse effect upon the heat exchange, and

Y.the amount of finally purified ethylene withdrawn as product varies, the improvement which comprises recycling, to said heat exchange system, that part of the finally puritied ethylene not withdrawn as product, and proportioning thus recycled ethylene, with respect to ethylene feed entering the process, to maintain substantially constant the total dlow rate of ethylene passing through said heat exchange system.

j References Cited in the tile of Ythis patent VUNITED STATES PATENTS 2,474,006 Maycock VJune 2l, 1949 2,500,353 Gantt ...e Mar. 14, 19,50 2,673,829 King Mar. 30, 1954 2,736,756 Elgin Feb, 28, 1956 2,781,293 RagatZ Feb. l2, 1957 2,804,488 Cobb Aug. 27, 1957 

1. IN A PROCESS WHEREIN ETHYLENE IS PURIFIED IN A PURIFICATION ZONE AND, IN CONJUNCTION WITH THE PURIFICATION, IS PASSED THROUGH A HEAT EXCHANGE SYSTEM WHEREIN IT IS SUBJECTED TO INDIRECT HEAT EXCHANGE WITH AT LEAST ONE MATERIAL WHICH HAS PASSED THROUGH SAID PURIFICATION ZONE, SO THAT VARIATION OF THE RATE OF FLOW OF ETHYLENE HAS AN ADVERSE EFFECT UPON THE HEAT EXCHANGE, AND THE AMOUNT OF FINALLY PURIFIED ETHYLENE WITHDRAWN AS PRODUCT VARIES, THE IMPROVEMENT WHICH COMPRISES RECYCLING, TO SAID HEAT EXCHANGE ZONE, THAT PART OF THE FINALLY PURIFIED ETHYLENE NOT WITHDRAWN AS PRODUCT, AND PROPORTIONING THUS RECYCLED ETHYLENE, WITH RESPECT TO ETHYLENE FEED ENTERING THE PROCESS, TO MAINTAIN SUBSTANTIALLY CONSTANT THE TOTAL FLOW RATE OF ETHYLENE PASSING THROUGH SAID HEAT EXCHANGE SYSTEM. 